The crust of the Earth moves when stress, or force, is
applied to it. Earthquakes are caused by stresses from plate movement, and
to a much lesser degree, from the movement of magma inside the Earth.
Understanding earthquakes teaches students about the inside of the Earth and
what causes visible movement on the outside surface of the Earth. Students
should understand that stress within the crust of the earth can
"relieve" itself by giving off energy (earthquakes). This energy
is released in the form of seismic waves. These waves make the whole Earth
ring like a bell and travel through the entire Earth. The movement of these
waves within the Earth’s crust can cause minor to major damage to
structures on the surface of the Earth, especially close to the origin of
the earthquake. The damage depends on the intensity of the original stress
and how it travels through the crust.
Earthquakes are caused by the sudden movement and
fracturing of rock masses along preexisting faults. A fault is a broken
surface within the Earth’s crust. The point on the fault at which the
displacement begins is called the "focus" of the earthquake. The
point on the surface of the earth directly above the focus is the
"epicenter." Your students need to understand that an earthquake
happens in rocks that have been stressed. This stress is stored until the
strength of the rock is exceeded. The actual break (the earthquake) then
releases the energy. Again, this energy travels in the form of waves.
The seismic waves generated by an earthquake can be
recorded and measured on a seismograph. The interpretation of the waves
provides seismologists with a way of "seeing" into the inside of
the Earth. The waves produced by earthquakes travel through the Earth and
bounce off different features of the Earth's interior. The patterns they
form after bouncing off these features can be used to create images of the
interior. The reflection of seismic waves indicates that the center of the
Earth is composed of iron and nickel. This core has two parts, the outer
core where the metal is liquid (not like milk, more like thick honey) and
the inner core, which is solid. Since we cannot drill very far into the
Earth’s crust, the evidence from different waves becomes important in
interpreting the earth's structures.
- State for students that volcanoes and earthquakes are related, but
that this unit will concentrate on earthquakes. Explain that the stress
within the crust of the Earth causes the Earth to "relieve"
itself in the form of earthquakes and volcanoes. Tell the class that
earthquakes can occur without volcanoes, but volcanoes are always
accompanied by earthquakes.
- On their worksheet, have the students trace the words. Emphasize to them that the entire Earth shakes during an
earthquake, not just one little area. Point out that an earthquake is
usually the strongest at the place where it starts, and that the farther
you get from the break in the Earth (focus - within crust; epicenter -
on the crust), the less you feel it. In the box, you may want students
to draw a "shaking" person or building.
- You might ask the students if any of them have ever felt an
earthquake. Let them tell their own stories. Remember that if one of the
students has had a bad experience, do not let that student continue too
long for it may scare the other children. Ask the students if they
thought of going to safety. Ask them if the earthquake was strong,
moderate, or weak, and what it felt like.